Refrigerator control apparatus

ABSTRACT

A refrigerator control apparatus comprising an electronic timer including first, second and third counters in which clock signals are applied to the input stage of the first counter, the output from the first counter is applied to the input stages of the second and third counters, the overload time of a compressor is controlled by a timing signal picked up from an intermediate stage of the first counter, the suspension time of the compressor is controlled by a timing signal picked up from the output stage of the second counter, the defrosting suspension time is controlled by a timing signal picked up from the output stage of the third counter, and the output pulses of the first counter are counted by the third counter only during the running of the compressor.

United States Patent Fujimoto et al.

REFRIGERATOR CONTROL APPARATUS Inventors: Ryoichi Fujimoto, Tochigi-ken;

Kazuo Takasugi, l-ligashiyamato; Fumiyuki lnose; Kenji Maio, both of Kokubunji; Norio Yokozawa, Fuchu, all of Japan Assignee: Hitachi, Ltd., Japan Filed: Aug. 12, 1974 Appl. No.: 496,897

[30] Foreign Application Priority Data References Cited UNITED STATES PATENTS 1/1962 Wechsler 62/156 Primary Examiner-Meyer Perlin Attorney, Agent, or Firm-Craig & Antonelli [5 7] ABSTRACT A refrigerator control apparatus comprising an electronic timer including first, second and third counters in which clock signals are applied to the input stage of the first counter, the output from the first counter is applied to the input stages of the second and third counters, the overload time of a compressor is controlled by a tfi'rning signal picked up from an intermediate stage of the first counter, the suspension time of the compressor is controlled by a timing signal picked up from the output stage of the second counter, the defrosting suspension time is controlled by a timing signal picked up from the output stage of the third counter, and the-output pulses of the first counter are counted by the third counter only during the running of the compressor.

'6 Claims, 5 Drawing Figures POWERSOLRCE POWER CONTRO L SOURCE CIRCUIT CIRCUIT SENSOR CIRCUIT 7 CLOCK FULSE GENERATOR F I G COIVPRESSOR DRIVING MOTOR PULSE CL GENERATOR CONTROL CIRCUIT FAN I SHEET SENSOR CIRCUIT SOURCE CIRCUIT FIG 2 FLIP FLIP FLP TIMER CIRCUIT FLOP FLOP FLOP D WER souRcE HEATER PATENTEIJJlI-II 24 I975 SHEEI PATENTEDJUM 24 I975 FIG T i) 43 42 :57 H 56 58 TOFF COUNTER 5 3i 55 7 l OOUNTER 7 rc -COUNTER TRT A B CP TR SLTRQSL FIG.

I REFRIGERATOR CONTROL APPARATUS I BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a refrigerator control apparatus employing an electronic timer for controlling the time of starting and stopping of the compressor, initiation and termination of a defrosting operation and other functions considered required of a refrigerator.

2. Description of the Prior Art A conventional widely used timer of this type is such that a timing signal is obtained by counting clock pulses derived from a signal of predetermined frequencies such as from a power supply of commercial frequencies, and such a timing signal is used to control various devices including a clock.

The timing control required of a refrigerator includes:

1. control of a time period during which an excessive current flows in the starting of the compressor (hereinafter referred to as the compressor overload time);

2. control of a time period from a time point when the compressor is stopped to a time point when it is reactuated (hereinafter referred to as the compressor suspension time);

3. control of a time period before starting of a defrosting process (hereinafter referred to as the defrosting suspension time); and

4. control of a maximum defrosting time permitted until completion of defrosting.

The periods of time involved in (l), (2), (3) and (4) above are on the order of several seconds, several minutes, several to several tens of hours and several tens of minutes respectively.

Even though a timer may be provided for each of the different types of timing controls, the multiplication of as many different types of timers as the types of timing signals involved is not desirable from the viewpoint of not only economy but reliability.

Further, the amount of frost grown on the cooling systemof the refrigerator is generally proportional to the running time of the compressor. The automatic starting of defrosting following a predetermined defrosting suspension time poses no problem as far as the starting and stopping of the compressor is effected at regular intervals of time. Irregular time intervals of compressor operation, however, which may be caused by a prolonged state of an open door or changes in the environmental temperature, leads to such an inconvenience that a defrosting operation is started long before or after a predetermined running time of the compressor, thus making it impossible to start the defrosting process at an optimum time point.

SUMMARY OF THE INVENTION A first object of the invention is to provide an economical and reliable refrigerator control apparatus which permits various types of timing control with a timer comprising only a few counters.

Another object of the invention is to provide a refrigerator control apparatus which is capable of optimum defrosting operation by accumulating the running time of the compressor.

In order to achieve the above-mentioned objects, there is provided according to the invention a refrigerator control apparatus comprising an electronic timer including first, second and third counters; in which clock pulses are applied'to the input stage of the first counter, the output from the first counter is applied to the input stages of the second and third counters, the compressor overload time is controlled by a timing signal picked up from a predetermined stage of the first counter, the compressor suspension time is controlled by a timing signal picked up from the output stage of the second counter, a maximum defrosting time is controlled by a timing signal picked up from a predetermined stage of the third counter, the defrosting suspension time is controlled by a timing signal picked up from the output stage of the third counter, and the output pulses of the first counter are counted by the third counter only during the running of the compressor.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a block diagram showing the refrigerator control appafatus according to an embodiment of the invention.

FIG. 2 is a block diagram showing an actual construction of the control circuit included in the apparatus of FIG. 1.

FIG. 3 is a block diagram showing an actual example of a more detailed construction of the control circuit of FIG. 2. i

FIG. 4 is a b lock diagram showing an example of the actual construction of the timer shown in FIG. 3.

FIG. 5 is a waveform diagram for explaining the operation of the timer of FIG. 4.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Referring to FIG. 1 showing a schematic diagram of an embodiment of the invention, reference numeral 1 shows a power source, numeral 2 a motor for driving a compressor which cools the space inside of the refrigerator, numeral 3a fan motor for circulating cool air inside of the refrigerator, and numeral 4 a defrosting heater for melting frost grown on the cooling system. Reference numerals 20, 30 and 40 show control elements comprising semiconductor switch elements for controlling the on-off operation of the compressor drive motor 2, fan motor 3 and defrosting heater 4 respectively. Numerals 21, 31 and 41 show signal lines for turning on andoff the control elements 20, 30 and 40 respectively. Reference numeral 5 shows a control circuit including an electronic timer, numeral 6 a power source circuit therefor, and numeral 7 a clock pulse generator circuit for applying clock pulses to the timer circuit. Numeral 8 shows a sensor circuit for applying a signal to the control circuit 5 and includes means for detecting temperatures in the refrigerator and current in the circuits.

The control operations in the embodiment under consideration are effected as follows:

1. When the compressor drive motor 2 is actuated by turning on the control element 20, the control circuit 5 detects the duration of the starting current (which is usually several times the normal operating current (which is usually several times the normal operating current) which is in turn detected by the overload sensor in the sensor circuit 8. If the duration of the starting current is longer than that under normal conditions, it is decided that the apparatus is in an overloaded condition so that the control element 20 is immediately turned off thereby to stop the compressor drive motor 2.

2. It is only after a predetermined period of time following the turning off of the control element 20 that the compressor drive motor 2 is actuated again. When after the lapse of the predetermined period of time the control element 20 is turned on and the compressor drive motor 2 is started again, the compressor drive motor 2 continues to run if it has been started normally. In the event that the starting current is abnormally long, by contrast, the control element 20 is turned off again thereby to stop the compressor drive motor 2, with the result that the compressor drive motor 2 is prohibited from being started until the lapse ofa predetermined period of time.

3. When, after the control element 20 is turned on, the inner space of the refrigerator is cooled to a predetermined temperature by the continuous operation of the compressor drive motor 2, the control element 20 is turned off and the compressor drive motor 2 is stopped in response to a signal produced by a temperature sensor in the sensor circuit 8. it is only after the lapse of a predetermined period of time following the turning off of the control element 20 that the compressor drive motor 2 is actuated again.

4. The operating time of the compressor drive motor 2, which is interrupted in accordance with the signal from the temperature sensor as mentioned in (3) above, is accumulated by the control circuit 5. When the accumulated time exceeds a predetermined length of time, say, 24 hours, the defrosting heater 4 is energized by the control element 40 to remove the frost formed on the cooling system.

5. Upon completion of the defrosting operation, the defrosting completion sensor in the sensor circuit 8 turns off the control element 40 and the defrosting heater 4 is turned off, so that the defrosting operation is finished and a normal operating condition is regained. In order to prevent a case in which a failure of the defrosting completion sensor causes a prolonged energization of the defrosting heater 4 to overheat the inner space of the refrigerator, the normal refrigerator operation is regained by the control element 5, which turns off the defrosting heater 4 after a predetermined period of time even in the absence of a signal from the defrosting completion sensor.

By the way, when the defrosting heater 4 is turned off and the compressor drive motor 2 is turned on, the control element 30 is turned on thereby to drive the fan motor 3.

It will be noted from the foregoing description that the control operations require a number of timing signals concerned with:

as to (l), the compressor overload time 'r for protecting the compressor from an overloaded condition,

as to (2) and (3), the compressor suspension time T which continues from the stoppage of the compressor to the next actuation thereof,

as to (4), the accumulated operating time or defrosting suspension time 1' before the starting of a defrosting process, and

as to (5), a maximum defrosting time TM.

These four types of timing signals are generated by the timer circuit 50 in the control circuit 5 shown in FIG. 2. In other words, application of clock pulses from the clock pulse generator circuit 7 to the timer circuit 50 causes timing signals to be produced from predetermined stages of the timer circuit 50. These timing signals combine with signals from the temperature sensor 81, the overload sensor 82 and the defrosting completion sensor 83 to control the flip-flops 22, 32 and 42 corresponding to the control elements 20, 30 and 40 respectively.

Referring to FIG. 3 showing an actual example of the construction of the control circuit of FIG. 2, R-S flipflops 22 and 32 and a trigger flip-flop 42 correspond to the compressor drive motor 2, fan motor 3 and the defrosting heater 4 of FIG. 1 respectively and apply outputs to drivers 23, 33 and 43 for driving the control elements 20, 30 and 40 respectively. Numerals 51, 52 and 53 show counters each of which comprises multiple stages of cascade-connected flip-flops, for example, ll, 1 l and 4 stages for the counters 51, 52 and 53 respectively. The clock pulses from the clock pulse generator circuit 7 are applied first to the first stage of the counter 51. The counter 51 produces a signal at an intermediate stage thereof and sends it out to the output line 54. Also, the counter 51 produces an output at the final stage and applies it through an output line 55 to the first stage of the counter 53 and, through the AND gate 71, to the first stage of the counter 52. The counter 52 sends out outputs to output lines 56 and 57 by way of its intermediate and final stages respectively. The counter 53 counts output pulses from the counter 51 and produces its own output by way of the output line 58. A signal from the reset input line 59 is adapted to be applied to the counter 53 to clear it. A signal from the temperature sensor 81 is applied through OR gate 24 to the reset terminal R of the R-S flip-flop 22 and changes to 1 state when the temperature in the refrigerator reaches a predetermined level, say, 22C. The signal from the overload sensor 82 is applied through the AND gate 25 and the OR gate 24 to the reset terminal R of the R-S flip-flop 22 and turns to l state when an overloaded condition of the compressor drive motor 2 is detected. The output signal from the defrosting completion sensor 83 is applied through the OR gate 43 to the reset terminal of the trigger flip-flop 42 and is changed to 1 state upon completion of a defrosting operation.

On the other hand, the signal on the output line 54 of the counter 51 is applied to an input terminal of the AND gate 25, while the output signal on the output line 56 of the counter 52 is applied through the OR gate 43 to the reset terminal R of the flip-flop 42 of trigger type. The output from the flip-flop 42 on the output line 57 is applied to the trigger terminal T of the flipflop 42 of trigger type. The signal on the output line 58 of the counter 53 is applied to the set terminal S of the R-S flip-flop 22, while the signal on the reset input line 59 of the counter 53 is obtained from the OR gate 24. These signals on the output lines 54, 56, 57 and 58 represent the compressor overload time L, miximum defrosting time TM, defrosting suspension time 1' and compressor suspension time T to be controlled respectively.

Output signals from the output terminals Q of the R-S flip-flop 22 and the flip-flop 42 of trigger type are applied through the OR gate 72 to an input terminal of the AND gate 71.

The output signal from the output terminal Q of the flip-flop 42 of trigger type is applied through the OR gate 24 to the reset terminal R of the R-S flip-flop 22.

The output signal from the output terminal Q of the R-S flip-flop 22 is applied to the set terminal S of the R-S flip-flop 32, while the output from output terminal Q of the flip-flop 42 of trigger type is applied to the reset terminal R of the RS flip-flop 32, so thatthe fan motor 3 shown in FIG. I is driven at the time of starting the compressor and is stopped at the time of starting defrosting. I

The control operation of the circuit shown in FIG. 3 will be explained in detail below. i v

First, let us consider a case in which the cooling operation is initiated by starting the compressor drive motor 2. The temperature in the refrigerator is higher than a predetermined level of, say, 22C, the frost formed remains unremoved and the output signals of the temperature sensor 81 and thedefrosting completion sensor 83 are in the state of O.

Therefore, at a given time when a signal is produced on the output line 58 of the counter 53 and applied to the set terminai of the R-S flip-flop 22, the flipflop 22 is set so that the control element shown in FIG. 1 is turned on through the driver 23 and signal line 21 thereby to start the compressor drive motor 2. If the compressor drive motor 2 is normal, a starting current flows for a short period (5 seconds or less), followed by a constant operating current, with the result that the compressor drive motor 2 enters a normal operation phase and begins to cool the inner space of the refrigerator. In the event that the compressor drive motor 2 is locked for some reasons or other including an overloaded condition of the compressor drive motor 2, on the other hand, a starting current several times as large as the operating current continues to flow. If the over load sensor 82 is arranged to produce a 1 signal in the presence of current several times as large as the operating current, it is possible to detect any overloaded condition from a logical product or theresultof application of the outputs from the overload sensor 82 and the output line 54 of the counter 51, that is, a signal representing the compressor overload time 1- In other words, even when the output state of the output line 54 changes to l after the lapse of several seconds, say, 5 seconds, the output of the AND gate changes to l if the output of the overload sensor 82 is l that is, if the overloaded condition is sustained, with the result that the R-S flip-flop 22 is reset thereby to st op the compressor drive motor 2. The compressor drive motor 2 is not restarted until the R-S flip-flop 22 is set as a result of change-over of the output line 58 of the counter 53 representing the compressor suspension time T to l state. In view of the fact that an overload time is determined on the basis of the output from the output line 54 of the counter 51, the information stored in the counter 51 is required to be zero at the very instant of starting the motor..lf each of the counters 51, 52 and 53 is provided with cascade-connected multiple stages of flip-flops as shown in FIG. 5 in such a way that the output of a given flip-flop changes its state at the time of the input fall, a change in the output state of a stage indicates output fall of all the preceding stages. In FIG. 5, clock pulses are shown by symbol CP and outputs of respective stages by A, B and C. For this reason, the information stored in the counter 51 is zero the very instant that the compressor drive motor 2 is started as a result of the R-S flip-flop 22 being set in response to a 1 signal produced on output line 58 of the counter 53, thus making it possible to produce a proper timing signal at the output line 54 without any special operations.

The counter5 3 counts the time, say, seven minutes during which the compressor drive motor 2 remains stationary, that is, the compressor suspension time 1 For this purpose, the counter 53 is cleared by the reset signal on the reset input signal line 59 leading to the R-S flip-flop 22.

Limiting the clearing operation to the counter 53 results in a' variation in the compressor suspension time equivalent 'to the information stored in the counter 51 at that time, thus affecting the temperature of the refrigerator inner space. If the compressor suspension time is lengthened, for example, the refrigerator temperature rises. However, changes in the temperature of the refrigerator inner space, which are compensated for in the next temperature controlcycle in accordance with the lengthened or shortened compressor suspension time, need not be controlled as strictly as the compressor overload time.

The counter 52 is provided for the purpose of accumulating the running time of the compressor drive motor 2. To achieve this purpose, outputs from the output terminals Q of the R-S flip-flop 22 and trigger flipflop 42 are vapplied through the OR gate 72 to an input terminal of the AND gate 71. As a result, when the output of the trigger flip-flop 42 is O, that is, when the refrigerator is not defrosting, output pulses from the counter 51 are counted by the counter 52, only if the output of the flip-flop 22 is I," that is, if the compressor drive motor 2 is running.

When the accumulated running time of the compressor drive motor .2, that is, the defrosting suspension time 1' reaches a predetermined value, say, 24 hours, the output line 57 of the counter 52 produces a l signal and the trigger flip-flop 42 is set. The output of the trigger flip-flop 42 resets the RS flip-flop 22 and turns on the defrosting heater 4 of FIG. 1 thereby to start a defrosting operation, while at the same time stopping the compressor drive motor 2. When the output of the defrosting completion sensor 83 changes to 1 state, the trigger flip-flop 42 is reset through the OR gate 43 thereby to end the defrosting operation. The defrosting completion sensor 83 comprises such an element as a thermistor and depends for its operation on the measurement of the temperature of the evaporator. In spite of the 0 state of the output of the RS flip-flop 22, the counter 52 continues its counting operation even during the defrosting operation because of the l state of the output of the trigger flip-flop 42.

Therefore, even if the defrosting completion senso 83 produces no 1 output after a prolonged period, the lapse of a predetermined period of time, that is, the maximum defrosting time 1 after the start of defrosting causes the counter 53 to produce a 1 output at its output line 56, so that the trigger flip-flop 42 is reset through the OR gate 43 thereby to stop the defrosting operation.

It will be understood from the above explanation that according to the present invention a plurality of types of timing signals required are obtained with a minimum number of counters, resulting in a great advantage for practical applications. The minimum required number of stages of counters is the one required for setting the maximum defrosting suspension time (about 24 hours for the present case, which is the accumulated running time of the compressor drive motor 2 before starting a defrosting cycle), which is set by the counters 51 and 52. Since the input to the compressor suspension time counter 53 is connected to the output of the counter 51, several stages or actually 3 or 4 stages in addition to the minimum required number of stages of counters 51 and 52 suffice for the purpose of the counter 53. Since only the counter 53 is cleared for the purpose of counting the compressor suspension time without clearing the counter 51, a small error occurs of the accumulated running time of the compressor drive motor 2 which is counted by the counter 52. However. it is as small as about 40 seconds and poses no problem for the proper control of the refrigerator.

Further, the apparatus according to the invention is so arranged that a defrosting cycle is started when a predetermined accumulated running time of the compressor drive motor 2 is reached, thereby making possible elimination of defrosting waste as well as optimum defrosting timing.

It will thus be seen that according to the invention the electronic circuit arrangement of a refrigerator control apparatus requiring a number of timing signals is most simplified on the one hand and an optimum defrosting operation is made possible by starting a defrosting cycle on the basis of the accumulation of the compressor running time, thereby leading to a great industrial value.

We claim:

1. A refrigerator control apparatus comprising: an electronic timer including first, second and third counters, a clock signal being applied to the input stage of said first counter, the output of said first counter being applied to the input stage of said second counter; means for picking up a first timing signal from a predetermined stage of said first counter; means for picking up a second timing signal from the output stage of said second counter; means for picking up a third timing signal from the output stage of said third counter; means for controlling the overload time ofa compressor in response to said first timing signal; means for controlling the suspension time of said compressor in response to said second timing signal; means for controlling the defrosting suspension time in response to said third timing signal; and means for applying the output of said first counter to said third counter during the running time of said compressor.

2. A refrigerator control apparatus according to claim 1, further comprising means for picking up a fourth timing signal from a predetermined stage of said third counter, means for controlling a maximum defrosting time in response to said fourth timing signal, and means for applying the output of said first counter to said third counter during the defrosting cycle.

3. A refrigerator control apparatus comprising: first, second and third counters each including a multiple of stages of flip-flops connected in cascade; means for applying a clock signal to the input stage of said first counter; means for applying the output from the output stage of said first counter to the input stage of said second counter; devices to be controlled including a compressor drive motor and a defrosting heater; first and second drive means for driving said compressor drive motor and said defrosting heater respectively; first and second flip-flops producing outputs for controlling said first and second drive means; a temperature sensor for detecting the reaching ofa predetermined temperature of the inner space of the refrigerator; an overload sensor for detecting an overcurrent in said compressor; a defrosting completion sensor for detecting the completion of a defrosting operation; a first OR gate impressed with the outputs from said first and second flip-flops; a first AND gate impressed with the output from the output stage of said first counter and the output of said first OR gate, said first AND gate applying its output to the input stage of said third counter; means for picking up a first timing signal from a predetermined stage of said first counter; means for picking up a second timing signal from the output stage of said second counter; means for picking up a third timing signal from the output stage of said third counter; a second AND gate impressed with the output of said overload sensor and said first timing signal from said first counter; a second OR gate impressed with the output of said second AND gate and the output of said temperature sensor, said second OR gate producingan output for resetting said first flip-flop and said second counter; means for setting said first flip-flop in response to said second timing signal from said second counter; means for setting said second flip-flop in response to said third timing signal from said third counter; and means for resetting said second flip-flop in response to the output from said defrosting completion sensor.

4. A refrigerator control apparatus according to claim 3, further comprising means for picking up a fourth timing signal from a predetermined stage of said third counter and a third OR gate impressed with said fourth timing signal and the output of said defrosting completion sensor, said third OR gate producing an out-put for resetting said second flip-flop.

"5. A refrigerator control apparatus according to claim 3, further comprising means for applying the output of said second flip-flop to said second OR gate.

6. A refrigerator control apparatus according to claim 3, further comprising a fan motor to be controlled, third drive means for driving said fan motor, a third flip-flop producing an output for controlling said third drive means, means for setting said third flip-flop in response to the output from said first flip-flop, and means for resetting said third flip-flop in response to the output of said second flip-flop. 

1. A refrigerator control apparatus comprising: an electronic timer including first, second and third counters, a clock signal being applied to the input stage of said first counter, the output of said first counter being applied to the input stage of said second counter; means for picking up a first timing signal from a predetermined stage of saId first counter; means for picking up a second timing signal from the output stage of said second counter; means for picking up a third timing signal from the output stage of said third counter; means for controlling the overload time of a compressor in response to said first timing signal; means for controlling the suspension time of said compressor in response to said second timing signal; means for controlling the defrosting suspension time in response to said third timing signal; and means for applying the output of said first counter to said third counter during the running time of said compressor.
 2. A refrigerator control apparatus according to claim 1, further comprising means for picking up a fourth timing signal from a predetermined stage of said third counter, means for controlling a maximum defrosting time in response to said fourth timing signal, and means for applying the output of said first counter to said third counter during the defrosting cycle.
 3. A refrigerator control apparatus comprising: first, second and third counters each including a multiple of stages of flip-flops connected in cascade; means for applying a clock signal to the input stage of said first counter; means for applying the output from the output stage of said first counter to the input stage of said second counter; devices to be controlled including a compressor drive motor and a defrosting heater; first and second drive means for driving said compressor drive motor and said defrosting heater respectively; first and second flip-flops producing outputs for controlling said first and second drive means; a temperature sensor for detecting the reaching of a predetermined temperature of the inner space of the refrigerator; an overload sensor for detecting an overcurrent in said compressor; a defrosting completion sensor for detecting the completion of a defrosting operation; a first OR gate impressed with the outputs from said first and second flip-flops; a first AND gate impressed with the output from the output stage of said first counter and the output of said first OR gate, said first AND gate applying its output to the input stage of said third counter; means for picking up a first timing signal from a predetermined stage of said first counter; means for picking up a second timing signal from the output stage of said second counter; means for picking up a third timing signal from the output stage of said third counter; a second AND gate impressed with the output of said overload sensor and said first timing signal from said first counter; a second OR gate impressed with the output of said second AND gate and the output of said temperature sensor, said second OR gate producing an output for resetting said first flip-flop and said second counter; means for setting said first flip-flop in response to said second timing signal from said second counter; means for setting said second flip-flop in response to said third timing signal from said third counter; and means for resetting said second flip-flop in response to the output from said defrosting completion sensor.
 4. A refrigerator control apparatus according to claim 3, further comprising means for picking up a fourth timing signal from a predetermined stage of said third counter and a third OR gate impressed with said fourth timing signal and the output of said defrosting completion sensor, said third OR gate producing an output for resetting said second flip-flop.
 5. A refrigerator control apparatus according to claim 3, further comprising means for applying the output of said second flip-flop to said second OR gate.
 6. A refrigerator control apparatus according to claim 3, further comprising a fan motor to be controlled, third drive means for driving said fan motor, a third flip-flop producing an output for controlling said third drive means, means for setting said third flip-flop in response to the output from said first flip-flop, and means for resetting said third flip-flop in response to the output of said second flip-flop. 